Field of the Invention
The present invention relates to a temperature-dependent switch with a housing that comprises a cover part with a lower side and an upper side as well as an electrically conductive lower part with a circumferential shoulder and a circumferential wall, whose upper section overlaps the cover part, with at least a first external contact surface arranged on the upper side of the cover part, at least a second external contact surface provided externally on the housing, wherein the upper section of the circumferential wall of the lower part that overlaps the cover part presses the cover part onto the circumferential shoulder, and with a temperature-dependent switching mechanism arranged in the housing which, depending on its temperature, establishes or opens an electrically conductive connection between the first and second external contact surfaces, wherein a sealing means is provided between the cover part and the lower part.
Related Prior Art
Such a switch is known from DE 196 23 570 A 1.
The known temperature-dependent switch is used, in a manner known per se, to monitor the temperature of a device. For that purpose it is, for example, brought into thermal contact through its external surfaces with the device to be protected, so that the temperature of the device to be protected affects the temperature of the switching mechanism.
The switch is connected electrically in series in the power supply circuit of the device to be protected by means of connecting wires soldered to its two external contact surfaces so that the supply current to the device to be protected flows through the switch when below the response temperature of the switch.
The known switch comprises a deep-drawn or turned lower part, in which an internal, circumferential shoulder is provided, on which a cover part rests. The cover part is held firmly against this shoulder through a circumferential raised wall of the lower part, whose upper section is folded radially inwards.
Since the cover part and the lower part are made of electrically conductive material, an insulating foil is provided between them, running around the cover part, extending inside the switch parallel to the cover part, and drawn up at the side, so that its edge region extends up to the upper side of the cover part. The folded upper section of the circumferential wall of the lower part thus lies on the edge region of the insulating foil.
The temperature-dependent switching mechanism here comprises a snap-action spring disk that carries a movable contact part, along with a bimetal disk put over the movable contact part. The snap-action spring disk presses the movable contact part against a stationary counter-contact inside on the cover part.
The snap-action spring disk is supported by its edge in the lower part of the housing, so that the electrical current flows from the lower part through the snap-action spring disk and the movable contact part into the stationary counter-contact, and from there into the cover part.
A first external contact surface, which is arranged in the center on the cover part, acts as a first external connection. A second external contact surface provided on the folded wall of the lower part acts as the second external connection. It is also, however, possible for the second external connection not to be arranged at this edge, but at the side on the current-carrying housing or on the lower side of the lower part.
Attaching a current transfer member on the snap-action spring disk in the form of a contact bridge that is pressed by the snap-action spring disk against two stationary counter-contacts provided on the lower side of the cover part is known from DE 198 27 113 C 2. In this case the second external contact surface is also arranged on the upper side of the cover part. The two counter-contacts are connected via the cover part with the two external contact surfaces. The current then flows from one external contact surface, via the associated counter-contact, through the contact bridge into the other stationary counter-contact, and from there to the other external contact surface, so that the operating current does not flow through the snap-action spring disk itself.
This design is in particular chosen when very high currents that no longer can be carried without problem through the spring disk itself have to be switched.
In both design variants, a bimetal disk, which lies force-free in the switching mechanism when below its critical temperature, is provided for the temperature-dependent switching function.
In the context of the present invention, a bimetal part refers to a multilayer, active, sheet-like component of two, three or four inseparably bonded components with different coefficients of expansion. The joins between the individual layers of metal or metal alloy are materially bonded or form-fitted, and are, for example, fabricated by rolling.
Bimetal parts of this kind have a first stable geometric configuration in their low-temperature position, and a second one in their high-temperature position, between which they jump, depending on the temperature, in a hysteresis-like manner. When the temperature changes above their response temperature or below their return temperature, the bimetal parts snap into the respectively other configuration. The bimetal parts are therefore often referred to as snap-action disks, and when seen from above can be elongated, oval or circular in form.
If, as a result of a rise in temperature in the device to be protected, the temperature of the bimetal disk now rises above the response temperature, the bimetal disk changes its configuration, and so acts against the snap-action spring disk in such a way that the movable contact part is lifted off the stationary counter-contact or the current-transfer member is lifted off the two stationary counter-contacts, so that the switch opens and the device to be protected is switched off and can no longer heat up.
In these designs, the bimetal disk is held without mechanical force when under its response temperature, and the bimetal disk thus also is not used to carry the current.
It is advantageous here that the bimetal disks exhibit a long mechanical service life, and that the switching point, that is the response temperature of the bimetal disks, also does not change even after a large number of switching operations.
When the requirements for the mechanical reliability and/or the stability of the response temperature are lower, the bimetal snap-action disk can also perform the function of the snap-action spring disk and, potentially, also of the current transfer member, so that the switching mechanism only comprises one bimetal disk, which then carries the movable contact part or comprises two contact surfaces instead of the current transfer member, so that the bimetal disk not only provides the closing pressure of the switch, but also, carries the current when the switch is in the closed state.
The provision of a parallel resistor, connected in parallel with the external terminals, to switches of this type is furthermore known. When the switch is opened, this parallel resistor takes part of the operating current, and holds the switch at a temperature above the response temperature, so that the switch does not automatically close again after cooling down. Switches of this sort are known as self-holding.
Fitting a series resistor, through which the operating current flowing through the switch passes, to switches of this type is furthermore known. In this way, an ohmic heat, proportional to the square of the current flowing, is generated in the series resistor. If the magnitude of the current exceeds a permitted size, the heat of the series resistor has the result that the switching mechanism is opened.
In this way, a device to be protected is already disconnected from its power supply circuit when an excessively high flow of current that has not yet resulted in excessive heating of the device is noted.
Instead of a usually circular bimetal disk, it is also possible to use a bimetal spring clamped at one end and supporting a movable contact part or contact bridge.
It is also, however, possible to use temperature-dependent switches which, as current transmission members, do not comprise a contact plate but rather a spring part which carries the two counter-contacts, or on which the two counter-contacts are formed. The spring part can be a bimetal part, in particular a bimetal snap-action disk, which not only implements the temperature-dependent switching function, but at the same time also provides the contact pressure and carries the current when the switch is closed.
All these different design variants can be implemented with the switch according to the invention; in particular the bimetal disk can perform the function of the snap-action spring disk.
A temperature-dependent switch, with a comparable construction to that of DE 196 23 570 A 1 referred to above is known from DE 195 17 310 A 1, in which the cover part, however, is made of a positive temperature coefficient thermistor material, and which can lie on a circumferential shoulder in the inside of the lower part without a layer of insulating foil being placed between them, against which it is pressed by the upper section of the circumferential wall of the lower part which is folded radially towards the inside.
In this way the positive temperature coefficient cover is connected in parallel with the two external terminals, so that it provides the switch with a self-holding function.
Positive temperature coefficient thermistors of this type are also known as PTC resistors. They are made, for example from semiconducting, polycrystalline ceramics such as BaTiO3.
The cover part of the temperature-dependent switch with contact bridge known from DE 198 27 113 C 2 referred to above is again made of positive temperature coefficient material, so that it also exhibits a self-holding function. Two rivets are arranged here on the cover part whose heads, lying on the outside, form the two external terminals, and whose heads on the inside interact as stationary counter-contacts with the contact bridge.
In a switch with this type of construction, the cover part can also be made of insulating material or of metal, where in the latter case, as in the switch known from DE 196 23 570 A 1, an insulating foil is provided, running around the cover part and extending within the switch parallel to the cover part and pulled upwards at the sides, so that its edge region extends up to the upper side of the cover part. The upper section of the circumferential wall of the lower part, which is folded radially inwards, here presses, with the insulating foil in between, onto the cover part.
In the known switches, the housing is usually protected against the ingress of contamination by a seal, which is applied before or after joining the connecting lugs or connecting cables to the external terminals.
Molding the external terminals with a single-component thermosetting plastic is known from DE 41 43 671 A 1. Casting the connecting lugs with an epoxy resin is known from DE 10 2009 039 948. It is also known that an impregnating varnish or protective varnish is frequently applied to the known switches after soldering to the connecting cables or connecting lugs.
To prevent the varnish penetrating here into the inside of the housing, the cover part of the switch known from DE 196 23 570 A 1 referred to at the outset is provided with a sealing means in the form of a circumferential bead which runs radially outside on the lower side of the cover part, and with which, when the upper section of the circumferential wall of the lower part is folded, the insulating foil is constricted. While this does provide better sealing, in many cases varnish nevertheless does penetrate into the inside of the housing.
In the comparable switches known from DE 196 23 570 A 1 mentioned at the outset, the insulating foil lying between the lower part and the cover part is pulled up to the side between the wall of the lower part and the cover part, and its edge region is turned up onto the upper side of the cover part. The stiff insulating foil becomes rippled by the turning over, and forms rosettes which cannot be reliably sealed by the upper section of the circumferential wall of the lower part that is pressed flat onto them. There is, moreover, a risk that the finishing varnish penetrates inside the switch through the rosettes. DE 196 23 570 A 1 attempts to reduce this problem through the bead that has already been mentioned.
DE 10 2013 102 089 B 4 describes a switch which, in principle, is known from DE 196 23 570 A 1 explained above. This switch comprises a spacing ring between the shoulder in the lower part and the cover part, which permits a larger contact gap between the movable contact part and the stationary counter-contact. To overcome the known sealing problem with the switch described in DE 196 23 570 A 1, the edge region of the insulating sheet in this switch is given V-shaped incisions from the outside, whereby the ripple is greatly reduced, so improving the sealing.
DE 10 2013 102 006 B 4 also describes a switch, as is known in principle from DE 196 23 570 A 1 explained above. This switch, like the switch known from DE 195 17 310 A 1 comprises a cover part of positive temperature coefficient material. Due to the poor resistance to compression of this PTC cover, the upper section, folded radially inwards, of the circumferential wall of the lower part cannot provide sufficient sealing in the known switch against the ingress of contamination, for which reason the folded upper section of the circumferential wall in the switch known from DE 195 17 310 A 1 must be sealed against the upper side of the cover part with silicone, which leads frequently to problems.
DE 10 2013 102 006 B 4 solves this problem in that a covering foil is provided which only lies on the upper side of the PTC cover, and into which the upper section of the circumferential wall of the lower part which is folded and lies flat against the covering foil, penetrates. The front side of the upper section of the circumferential wall faces away from the covering foil. The upper section of the circumferential wall of the lower part, which is lying flat, however frequently does not provide the desired sealing.
A covering foil and an insulating foil can also be provided to a switch, as is illustrated, for example, by DE 10 2013 102 089 B 4. An insulating covering foil, for example made of Nomex®, is arranged on the upper side of the cover part of this switch, extending with its edge radially outwards as far as the insulating foil, which consists, for example, of Kapton®. Nomex® and Kapton® consist of aramid paper and of aromatic polyimides, respectively.
In spite of the various sealing measures, sealing problems continue to occur with the known switches, due in part to the fact that, as a result of the bending of the upper section of the circumferential edge of the lower part, the relatively stiff insulating foils cannot achieve a lasting seal. In addition, the cost of the construction that is necessary in order to achieve good sealing is high.